There have been several proposals to meet the ever increasing traffic demands and high quality expectations for mobile broadband services. Some of the most widely adopted means to meet these demands include an upgrading of existing base stations to use higher data rate technologies such as High Speed Packet Access (HSPA) or Long Term Evolution (LTE), or to use other optimizations such as Multiple Input Multiple Output (MIMO), antenna tilting, etc. Mobile broadband systems can be further enhanced by a straightforward increase in the number of base stations in a given network, a technique known as macro densification. However, these methods of improving the data rate can provide system gains only to a certain extent, and they can end up being very expensive.
As a result, the concept of “heterogeneous networks,” where the existing homogeneous network is overlaid with additional lower-power, low-complexity base stations, is currently being developed by members of the 3rd-Generation Partnership Project (3GPP) as a solution to mitigate the cost and/or capacity limitations of macro densification and base station upgrades.
The homogeneous layer of macro cells is known as a “macro” layer, as the base stations in this layer (known as eNBs, in 3GPP documentation for LTE) have large coverage areas. A non-homogenous layer overlaying the macro layer may contain several lower power nodes of any of several types. Some of these nodes are commonly referred to as “pico nodes” or “picos,” and are low power base stations for indoor or outdoor usage, e.g., for “hot spot” coverage in high-traffic areas. Other nodes, typically transmitting with even lower power, are often referred to as “femto nodes” or simply “femtos.” These are often used as home base stations (HeNBs), and are usually for indoor usage, such as in a private residence or small shop. Femtos that are open to only a few users (within a household, a shop, etc.) are termed within 3GPP as utilizing a Closed Subscriber Group (CSG).
Heterogeneous networks are expected to offer a low cost alternative to macro densification and will likely be more effective, since the deployment of the low power nodes can focus on traffic hot spots and areas with coverage problems. Note that the term “small cell” is used to refer to a pico or a femto cell for the rest of this document. Also, references to an eNB in the discussion that follows should be understood to refer to a macro eNB supporting a macro cell, unless the eNB is specifically referred to as a low-powered eNB supporting a small cell.
Handover is an important aspect of mobile communication systems, wherein the system tries to assure service continuity of a mobile terminal, known as “User Equipment” or “UE” in 3GPP documentation, by transferring the connection from one cell to another depending on several factors such as signal strength, load conditions, service requirements, etc. The provision of efficient/effective handovers, e.g., involving a minimum number of unnecessary handovers, a minimum number of handover failures, minimum handover delays, etc., affects not only the Quality of Service (QoS) of the end user but also the overall mobile network capacity and performance.
In LTE systems, UE-assisted, network controlled handover is utilized (see 3GPP TS 36.300, available at www.3gpp.org). The network configures the UE to perform measurements and send measurement reports when certain criteria are met. Based on these reports the UE is moved, if required and possible, to the most appropriate cell that will assure service continuity and quality. A UE measurement report configuration consists of the reporting criteria (whether it is periodic or event triggered) as well as the measurement information that the UE has to report.
In LTE, the most important measurement metrics used are the Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ). RSRP is a cell specific measure of signal strength and it is mainly used for ranking different cells for handover and cell reselection purposes, and it is calculated as the linear average of the power of the Resource Elements (REs) which carry cell-specific Reference Symbols (RSs). The RSRQ, on the other hand, also takes the interference into consideration by taking the total received wideband power into account as well.
One of the measurement configuration parameters that UEs receive from their serving eNBs is the “s-Measure,” which tells the UE when to start measuring neighboring cells. The s-Measure is defined as a Reference Signal Received Power (RSRP) value. Once a UE's measurement of the RSRP of its serving cell drops below the s-Measure threshold, the UE begins measuring the RSRP of neighboring cells, and the measured neighboring cells may ultimately be used for cell re-selection through handover. The s-Measure is an optional parameter, per the 3GPP specifications, and different s-Measure values can be specified for initiating intra-frequency, inter-frequency and inter-RAT measurements.
In homogenous networks, the use of the s-Measure as described above works quite well, because cells are usually deployed in such a way that they have small shared coverage areas around their cell edges. When a UE is very close to the eNB, the RSRP is typically higher than the s-Measure, and when the UE moves towards the cell edge, the RSRP decreases. At some point it falls below the s-Measure and the UE starts measuring the Reference Signal (RS) from neighboring cells. Thus, the UE starts measuring RS from other cells only when it is necessary to do so.
In heterogeneous networks, on the other hand, a straightforward usage of the s-Measure may cause problems, because small cells may be located close to the macro eNB (e.g., for capacity boosting in hotspots), so that the RSRP of the serving cell may be strong enough to not fall below the s-Measure threshold near the coverage area of the small cell, causing the UE not to measure the signal power of otherwise available small cells within its macro serving cell.